After weeks of gleefully watching Republicans struggle to find their footing in fiscal cliff talks, it’s time for the Democrats to do some painful soul-searching of their own.

President Barack Obama’s latest offer to congressional Republicans crosses lines that Democrats have long portrayed as untouchable. The provision causing the most heartburn for Democrats on Capitol Hill is one that would change the way inflation is measured to ultimately reduce payments to Social Security beneficiaries.

Obama floated the so-called chained consumer price index idea to House Speaker John Boehner (R-Ohio) last year during their failed negotiations over raising the debt ceiling. But by including it in his fiscal cliff offer, Obama is guaranteeing that if and when a deal comes together, it’s almost certain to include the provision.

It would force Democrats — who have spent decades building their brand as the protectors of Social Security and entitlement programs — into a difficult vote. And their support for a fiscal cliff package will be crucial to offset Republicans who might oppose any deal that raises tax rates, an area where Boehner has moved closer to Obama in recent days.

The White House dispatched Legislative Affairs Director Rob Nabors to Capitol Hill on Tuesday to explain the proposal to House and Senate Democrats and try to assuage their concerns.

Still, multiple lawmakers expressed deep concern about the direction of negotiations with less than two weeks left before Washington plunges off the fiscal cliff.

“This should be off the table,” a visibly frustrated Rep. Jan Schakowsky (D-Ill.) told reporters. “I hope that offer … will be reconsidered.”

Sen. Sherrod Brown (D-Ohio) was more blunt, calling the chained CPI proposal “terrible.” He said the existing inflation calculation was already insufficient.

“I don’t think the CPI now captures health care costs of retirees,” Brown said. “It’s more connected to people in their work lives with a different set of needs.”

New York Rep. Charles Rangel, a senior Democrat on the tax-writing Ways and Means Committee, called the chained CPI measure a “very, very bad idea.”

“It doesn’t save us much money, and it’s going to cause financial heartbreak,” he told POLITICO.

The White House estimates that the new inflation calculation would save $130 billion over 10 years. Most of those savings would probably occur in later years.

AARP, the nation’s largest lobbying organization on behalf of seniors, also urged Congress and the White House to nix the CPI change.

“This dramatic benefit cut would push thousands more into poverty and result in increased economic hardship for those trying desperately to keep up with rising prices,” Nancy LeaMond, executive vice president of AARP’s State and National Group, said in a statement.

Several Democrats said they’d keep resisting such changes.

“I think most of us in there are arguing very strongly against cuts to social safety net programs,” Connecticut Rep. Chris Murphy, who will join the Senate next year, said of fellow House Democrats. “I for one understand there’s likely going to be a tough vote for a lot of us on programs we really care about.”

And Massachusetts Rep. Jim McGovern said the provision does little more than “balance the budget on the backs of poor people.”

“Reducing the amount that somebody might get in terms of a cost-of-living adjustment for Social Security — people who are living on small checks — doesn’t seem like a very fair way to do it,” he said.

It’s doubtful the indignation will have much effect in the fast-moving negotiations, however.

White House spokesman Jay Carney defended including chained CPI in the president’s proposal and described it as an effort to find common ground with Republicans. And the White House has cast its version of the provision as “superlative CPI,” arguing that it protects particularly vulnerable groups that would be hit by Republicans’ preferred chained CPI plan.

And for all the angst in the Democratic Caucus, House Minority Leader Nancy Pelosi (D-Calif.) is expressing confidence that she can get the votes needed to pass Obama’s overall plan.

So far, immediate, fundamental changes to Medicare seem to be off the table. And some deficit-weary Democrats say the CPI proposal is necessary to get a deal.

“I’m strongly supportive of chained CPI,” North Dakota Sen. Kent Conrad told POLITICO. “It makes perfect sense to do. It affects both the revenue and the spending side in a way that I think is balanced and fair.”

Conrad might be in the minority among his colleagues.

Multiple senators leaving the closed-door meeting said Nabors didn’t delve into details, but many were reluctant to support certain provisions in the White House offer.

Sen. Joe Lieberman (I-Conn.) said there was “some consternation” in the caucus lunch Tuesday over the president’s proposal and said that while he personally was encouraged by Boehner and Obama moving closer together, not every Democrat was on the same page.

“I’m going to fight hard to keep Social Security out of this,” said Sen. Ben Cardin (D-Md.).

Rep. Keith Ellison (D-Minn.), co-chairman of the Congressional Progressive Caucus, noted that 102 Democrats recently signed a letter urging negotiators to keep Social Security off the table.

“Any agreement brought to the House floor is going to need many of those same Democrats to pass, so it will be difficult to pass something that includes a cut for those who use every dollar from their Social Security checks to pay for food and housing,” Ellison said.

Asked whether she could support the chained CPI measure in the White House offer, Sen. Claire McCaskill (D-Mo.) said: “It depends on if they’re protecting the most vulnerable.”

Outside liberal groups are keeping up pressure on Democrats to ward off a deal that includes any changes to Social Security.

Adam Green, the co-founder of the Progressive Change Campaign Committee, threatened primary challenges for Democratic lawmakers flirting with supporting the CPI change and warned that progressives were already getting mobilized to lobby members.

“People are under the kind of delusional idea that the American public would applaud cutting their own grandparents’ Social Security benefits because compromise is just that important,” Green said in an interview. “Well, it’s not. There’s a good compromise and a bad compromise.”

One measure that would help keep Democrats at the table is an extension of emergency unemployment benefits scheduled to lapse at the end of the year unless Congress acts. Obama’s proposal includes the benefits, though Republicans have previously tried to block extensions.

Democrats said the continued jobless benefits are needed as a stimulus for the economy, especially because Obama isn’t pressing for a continuation of the payroll tax cut. The end of that holiday will result in a 2-percentage-point tax increase for most middle-income workers starting in January.

And though most of the public battle over the fiscal cliff has centered on whether tax rates should increase for top earners, there appears to be little anger at Obama’s moderating position on the issue. After campaigning since 2008 to allow the Bush-era tax rate to expire for those earning more than $250,000, Obama’s latest proposal moves that threshold to $400,000.

Some Democrats representing high-income districts cheered the move.

“For some of my constituents, it makes a difference,” said Pennsylvania Rep. Allyson Schwartz, whose district encompasses some of Philadelphia’s wealthy suburbs. “A little movement on that is OK.”